1. Field of the Invention
This invention relates to a process for production of syn-gas (hydrogen, carbon monoxide) by feeding biomass and biomass-derived molecules (including sugars, starches, cellulose, hemicellulose, lignin, glycerol, and triglycerides) with water or CO2 into the regenerator of a fluid catalytic cracking (FCC) unit.
The European Commission has set a goal that by 2010, 5.75% of transportation fuels in the EU will be biofuels. Blending biofuels with petroleum based fuels will reduce dependence on imported crude oil, reduce emissions of greenhouse gases, and improve agricultural economies.
2. Background Art
Methods for conversion of solid biomass into liquids by acid hydrolysis, pyrolysis, and liquefaction are well known (Klass 1998). A wide range of products are produced from the above reactions including: cellulose, hemicellulose, lignin, polysaccharides, monosaccharides (e.g. glucose, xylose, galatose), furfural, polysaccharides, and lignin derived alcohols (coumaryl, coniferyl and sinapyl alcohols). Bio-oils, which are a mixture of over 300 different compounds, can also be produced by liquefaction or fast pyrolysis (Elliott, Beckman et al. 1991).
Fluid catalytic cracking (FCC) is the most widely used process for the conversion of crude oil into gasoline and other hydrocarbons. The FCC process consists of two vessels coupled together as shown in FIG. 1. In the first reactor a hot particulate catalyst is contacted with hydrocarbon feedstocks in a riser reactor to crack the feedstock, thereby producing cracked products and spent coked catalyst. After the cracking reaction takes place the catalyst is largely deactivated by coke. The coked catalyst is separated from the cracked products, stripped of residual oil by a series of baffles in a downflow reactor by steam stripping, and then regenerated by burning the coke from the coked catalyst in a regenerator. The regeneration process occurs at 650-760° C. and a pressure around 3 atm to burn off coke. The hot catalyst is then recycled to the riser reactor for additional cracking.
A variety of process configurations and catalysts have been developed for the FCC process. FCC catalysts usually contain mixtures of a Y-zeolite within a silica-alumina matrix although other compositions are also known to those skilled in the art. Using FCC processes for biomass conversion does not require a significant capital investment as FCC plants are already installed in petroleum refineries. It would therefore represent a considerable advance in the state of the art if efficient methods were developed to use the FCC process to convert biomass-derived molecules into fuels and chemicals.
Several methods have been reported for conversion of biomass-derived molecules into liquid fuels using zeolite catalysts. Chen and Koenig in U.S. Pat. No. 4,933,283 and U.S. Pat. No. 4,549,031 (Mobil) report a process for conversion of biomass derived carbohydrates, starches and furfural into liquid hydrocarbon products, CO and coke by passing aqueous streams over zeolite catalysts at 500° C. (Chen, T. F. Degnan et al. 1986; Chen and Koenig 1990). They observed that 40-66% of the carbon converts to coke when xylose, glucose, starch and sucrose are fed over a ZSM-5 catalyst at 500° C. (Chen, T. F. Degnan et al. 1986). Other products formed included hydrocarbons, CO and CO2. They also report that mixing the aqueous-carbohydrate streams with methanol leads to lower levels of coke and higher levels of hydrocarbons.
Bio-oils, produced by fast pyrolysis or liquefaction from biomass, are a mixture of more than 300 compounds. Bio-oils are thermally unstable, and need to be upgraded if they are to be used as fuels. Bio-oils, and bio-oil components, can be converted into more stable fuels using zeolite catalysts (Bridgwater 1994). Reaction conditions used for the above process are temperatures from 350-500° C., atmospheric pressure and gas hourly space velocities of around 2. The products from this reaction include hydrocarbons (aromatic, aliphatic), water soluble organics, water, oil soluble organics, gases (CO2, CO, light alkanes), and coke. During this process a number of reactions occur, including dehydration, cracking, polymerization, deoxygenation, and aromatization. However poor hydrocarbon yields and high yields of coke generally occur under reaction conditions, limiting the usefulness of zeolite upgrading.
Bakhshi and co-workers studied zeolite upgrading of wood derived fast-pyrolysis bio-oils and observed that between 30-40 wt % of the bio-oil formed coke or char. (Sharma and Bakhshi 1993; Katikaneni, Adjaye et al. 1995; Adjaye, Katikaneni et al. 1996) The ZSM-5 catalyst produced the highest amount (34 wt % of feed) of liquid organic products of any catalyst tested. The products in the organic carbon were mostly aromatics for ZSM-5, and aliphatics for SiO2—Al2O3. Gaseous products included CO2, CO, light alkanes, and light olefins. Bio-oils are thermally unstable and thermal cracking reactions occur during zeolite upgrading. Bakhshi and co-workers also developed a two reactor process, where only thermal reactions occur in the first empty reactor, and catalytic reactions occur in the second reactor that contains the catalyst. (Srinivas, Dalai et al. 2000) The advantage of the two reactor system is that it improves catalyst life by reducing the amount of coke deposited on the catalyst.
The transformation of model bio-oil compounds, including alcohols, phenols, aldehydes, ketones, acids, and mixtures, have been studied over HZSM-5 catalysts. (Gayubo, Aguayo et al. 2004; Gayubo, Aguayo et al. 2004; Gayubo, Aguayo et al. 2005) Alcohols were converted into olefins at temperatures around 200° C., then to higher olefins at 250° C., followed by paraffins and a small proportion of aromatics at 350° C. (Gayubo, Aguayo et al. 2004) Phenol has a low reactivity on HZSM-5 and only produces small amounts of propylene and butanes. 2-Methoxyphenol also has a low reactivity to hydrocarbons and thermally decomposes generating coke. Acetaldehyde had a low reactivity on ZSM-5 catalysts, and it also underwent thermal decomposition leading to coking problems (Gayubo, Aguayo et al. 2004). Acetone, which is less reactive than alcohols, converts into C5+ olefins at temperatures above 350° C. These olefins are then converted into C5+ paraffins, aromatics and light alkenes. Acetic acid is first converted to acetone, and that then reacts as above. Products from zeolite upgrading of acetic acid and acetone give considerably more coke than products from alcohol feedstocks. The majority of biomass derived molecules produce large amounts of coke when passed over acidic zeolite catalysts. It would therefore represent a considerable advance in the state of the art if efficient methods for conversion of biomass derived coke into premium products were developed.
When petroleum feedstocks contain levels of Conradson carbon greater than 6.0 wt %, special modifications are required in the FCC regenerator. In the 1980's Hettinger et al. from Ashland Oil published two patents on an FCC process for conversion of petroleum feedstocks with high levels of Conradson carbon (greater than 6.0 wt %) (Hettinger, Kovach et al. 1984; Hettinger, Kovach et al. 1984; Hettinger 1999). These feedstocks lead to high carbon levels and the temperature in the regenerator would rise to above 850° C. if high levels of carbon were present on the FCC catalyst. If the temperature in the regenerator is above 850° C. a rapid loss of catalytic activity and selectivity occurs due to loss of zeolite structure. To overcome these problems they proposed to reform some of the coke with CO2 in the catalyst regenerator. The CO2 reacted with the coke to form CO and H2O. They envisioned that this process could be used to also reduce the large amounts of CO2 emitted during the FCC process (Hettinger 1999). They proposed a two stage regenerator system, where CO2 would be used in a first stage to remove most of the hydrogen on the coke, and some carbon, and with possibly a second regenerator which would release enough heat for the cracking reaction. The CO2 from the second step would be recycled to the first stage. Hettinger et al. claimed that this process can be used to recycle feeds with Conradson carbon values up to 20 wt %.
In the patent they state that CO2 reforming could be done at temperatures from 715-800° C. The CO2 reforming reaction is endothermic, which decreases the temperature in the regenerator. In the patent they claim that the CO2 reforming ability of the catalyst can be improved by addition, at a range from 0.5-5 wt %, of the following metals, oxides or salts: Li, Na, K, Sr, V, Ta, Mo, Re, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, Au, Sn and Bi. To test the ability of different additives for CO2 reforming, they added the above metals at a 1 wt % level to a zeolite catalyst. They then coked the catalyst to a 1.1 wt % level, passed CO2 over the coked catalyst at 746° C. for 20 min and measured the coke levels. They then grouped the metals according to high activity (45-60% coke conversion), intermediate activity (30-35% coke conversion) and low activity (10-25% coke conversion). The catalysts with high activity included: Li˜Na>Re˜Fe˜Co˜Ni˜Ru˜Rh˜Pd˜Os˜Ir˜Pt>Cu˜Ag˜Au˜Sr. The catalysts with intermediate activity included: V>Sn˜Bi˜Mo. The metal catalysts with the lowest activity included: Ti˜Zr˜Hf˜Cr˜W˜Actinide Series>K˜Rb˜Cs˜Mg˜Ca˜Ba˜Sc˜Y˜La˜Mn˜Zn˜Cd˜Hg˜B˜Ga˜In˜Lanthanide Series>As˜Sb˜Se˜Te. Equilibrium FCC catalysts, in which metals were deposited during the FCC process, were also tested and had a high activity in CO2 reforming.
Steam reforming has also been reported as a method of regenerating coked FCC catalysts. The first mention in the patent literature for steam reforming of coke in an FCC reactor to produce syn-gas appears in 1950 as U.S. Pat. No. 2,518,775 assigned to Phillips Petroleum (Guyer 1950). The experimental section of this patent contains two experiments where a coked FCC catalyst was regenerated at 650° C. with air or with a steam/oxygen mixture. The outlet gas from the catalyst regenerated with air contained primarily N2, CO2, CO and O2. The outlet gas from the catalyst regenerated with the steam-oxygen mixture contained 38% CO2, 30% CO, and 33% H2 (% in volume). This patent then recommends that the syn-gas be used to produce alkanes by Fischer-Tropsch Synthesis. The authors recommended that catalyst regeneration be done at the highest temperature permissible without causing permanent catalyst deactivation. They recommended a temperature range of 540-980° C. and a regeneration gas of 10-80 vol. % oxygen and 90-20 vol. % steam. This is a steam to oxygen ratio between 9:1 to 1:4.
In the 1980s several more patents appeared in the literature for steam reforming of carbon in an FCC catalytic regenerator. Ralph W. Bradshaw from Phillips Petroleum patented a process to produce syn-gas in an FCC regenerator by adding steam to an air or oxygen stream that was fed into an FCC regenerator (Bradshaw 1980). This syn-gas was then sent to a water-gas shift reactor to generate hydrogen to be used for hydrocracking and other process operations. No experimental results were presented in this patent.
Another patent, assigned to Exxon, was written by Grenoble and Weissman in the early 80's (Grenoble and Weissman 1981). They tested catalysts consisting of oxides of tungsten or niobium combined with tantalum, hafnium, chromium, titanium and zirconium supported on Al2O3. They tested the catalytic activity for FCC in a fixed bed-reactor, and claim two different ways of catalytic regeneration: (1) partial combustion to produce a low BTU gas rich in CO and (2) steam addition to produce a gas rich in H2. The patent did not contain any experiments with regard to syn-gas production from coke. The goal of this patent was to reform feedstocks that contained high amounts of Conradson carbon, which leads to high coke levels on the catalyst surface. The inventors' approach was to develop novel materials that maintained their structures under high temperature steam treatments. In this patent they claim to have developed novel FCC catalysts that were stable at temperatures up to 760-1500° C., where traditional FCC catalysts suffer due to irreversible deactivation.
Another more recent patent written in 1994 by Hsing and Mudra from Texaco discussed ways to convert the carbon on an FCC catalyst to a gaseous mixture of CO2, CO, CH4, and H2 by steam reforming (Hsing and Mudra 1994). They report reaction conditions of temperatures from 540-650° C., steam to carbon molar ratios of 0.5-20 and reaction times from 0.5 to 30 minutes. They report that 10-40% of the carbon can be removed by these steam treatments. The remaining carbon is removed from the catalyst by a 2nd regeneration treatment with air. In a fluid catalytic pilot plant steam was injected into the steam stripper in the absence of oxygen at temperatures from 590-650° C. The off gas contained large amounts of H2 (32-50 vol %), CO2 (10-19%) as well as CO, CH4 and H2S. Removal of part of the carbon in the steam stripper reduced the temperature in the regenerator.
Although several processes have been suggested for converting coke deposits on FCC catalysts to syn-gas type compositions, no attempts have been reported to provide an external carbon source to FCC regeneration processes. In particular, the prior art contains no suggestion that an oxygen-rich carbon source, such as derived from biomass, may be used for this purpose.